Respectfully, that’s really not true at all.
I don’t think there’s a single person on this thread/in the community that only knows about pre-Disney Boba Fett based on his appearance in the OT. The truth is that there are numerous books and comics written about him/with him that elaborated on the character greatly, all the way from the Holiday Special up through 2013 pre-acquisition.
By your logic, and only counting movie screen time, most of the characters really never go beyond wooden, 1-dimensional tropes. Again, I find it hard to believe that there’s really anyone on this thread that legitimately holds to the belief that Boba has just been this brief cameo background character until Mando/BoBF. It isn’t an issue of head canon, there was a plethora of EU and Disney canon that fleshed this character out before the series.